The present invention relates to a catalyst for synthesizing hydrocarbons from a synthesis gas containing hydrogen and carbon monoxide as main components and a process for producing hydrocarbons using such a catalyst.
Reactions for synthesizing hydrocarbons from a synthesis gas containing hydrogen and carbon monoxide as main components are referred to as “Fischer-Tropsch synthesis (FT synthesis)” and are well-known in the related art. Fischer-Tropsch synthesis is carried out using catalysts obtained by loading an active metal such as iron or cobalt on a support such as silica or alumina (for example, see Patent Document 1 below).
It is reported that these catalysts are improved in carbon monoxide conversion (CO conversion) by the use of other metals (hereinafter referred to as “second metal”) in combination with such an active metal (for example, see Patent Document 2 below).
(1) Patent Document 1: Japanese Patent Laid-Open Publication No. 4-227847
(2) Patent Document 1: Japanese Patent Laid-Open Publication No. 59-102440
The FT synthesis reaction is defined by indexes such as CO conversion, methane selectivity, and chain growth probability α. A lower methane selectivity means that methane-generating reaction, i.e., a side reaction of the Fischer-Tropsch reaction, is suppressed low. The chain growth probability α is used as a measure of the molecular weights of the resulting hydrocarbons, and a higher chain growth probability a, i.e., close to 1.0 means that higher molecular weight hydrocarbons can be produced.
FT synthesis products are usually hydrocracked in the subsequent stages and then manufactured as clean liquid fuels. In recent years, among such liquid fuels, middle distillates such as kerosene and light gas oil are particularly in higher demand. In order to enhance the yield of these middle distillates, lower methane selectivity and higher chain growth probability α are required. Therefore, development of FT synthesis reactions of higher CO conversion, lower methane selectivity and higher α is held up a target in the industry and has been promoted by improving FT synthesis catalysts.
However, it is generally known that the CO conversion and chain growth probability α tend to be in a trade-off relation. When the reaction temperature is raised, the CO conversion increases but the chain growth probability α decreases. When the reaction temperature is lowered, the chain growth probability α increases but the CO conversion decreases. More specifically, a catalyst with a high chain growth probability α in a high CO conversion region has not been developed yet. This is the biggest obstacle in utilizing FT synthesis and a process for producing clean liquid fuels using the synthesis practically. For example, it is disclosed in Patent Document 2 hat CO conversion can be increased by loading a second metal such as zirconium or titanium on silica. However, in the case where a second metal is added in a large amount of, for example, 20 percent by mass by a conventional impregnating method, it causes harmful effects that pores of the silica support are plugged and thus the surface area thereof is decreased, resulting in the decrease of the chain growth probability.